DESCRIPTION: The development of adult T cell leukemia (ATL) correlates with HTLV-1 infection at the perinatal or postnatal stage in humans and the thymus may be a critical target organ in the establishment and maintenance of infection. HTLV can productively infect human hematopoietic progenitor (CD34+) cells and these stem cells transmit viral infection when implanted into the human thymus engrafted in SCID-hu mice. The goal of this proposal is to characterize the events involved in HTLV infection in the thymus and to establish an in vivo model of viral leukemogenesis. Additionally, the HTLV tax1 or tax2 oncogenes will be transduced into hematopoietic stem cells, using retroviral expression vectors, to evaluate the effect of Tax on the differentiation of hematopoietic stem cells in vitro and in vivo. It is hypothesized that the establishment of persistent HTLV-1 or -2 infection and/or tax gene expression in CD34+ cells and prothymocytes results in the perturbation of lymphopoiesis and that this is a predisposing factor for leukemogenesis. The Specific Aims of this proposal are: 1. Establish HTLV-1 and -2 infection in human hematopoietic progenitor (CD34+) cells and characterize the effects on hematopoiesis. HTLV infection of CD34+ cells will be analyzed to quantify the effect on the differentiation of stem cells in vitro and to determine if viral gene expression is maintained in differentiated progeny cells. 2. Study the effect of HTLV infection on T-Iymphopoiesis in vivo. Reconstitution of SCID-hu mice with infected CD34+ cells will determine if establishment of high levels of HTLV can perturb the development of T cells in the thymus and whether these mice develop leukemia. 3. Determine the role of tax expression on the differentiation of hematopoietic progenitor cells in vitro and on transformation of human thymocytes in vivo. High-efficiency transduction of the tax genes into CD34+ cells will be accomplished by pseudotyping retroviral expression vectors with VSV-G envelope and selecting transduced stem cells to test whether Tax can transform human thymocytes in vivo.